1. Field of the Invention
This invention relates to an intake air control apparatus for an internal combustion engine mounted in an automotive vehicle or the like or, in particular, to an electronically controlled throttle control unit for driving a drive motor in accordance with the accelerator pedal angle set by the driver and thus controlling the rotational angle of the throttle valve of butterfly type rotatably accommodated in the throttle bore of the throttle body.
2. Description of the Related Art
In the prior art, an electronically controlled throttle control unit having the opener-side function has been proposed (see, for example, U.S. Pat. No. 5,492,097, pp. 1–9, FIGS. 1 to 9) wherein, in the case where the electric current to the drive motor is stopped for some reason, the throttle valve is kept at a predetermined intermediate position (also called the default opening degree of the throttle valve) between the closed-up position and the full-open position by use of different forces of a plurality of springs, thereby making possible a movement of the automotive vehicle to a refuge without immediately stopping the internal combustion engine.
In the conventional electronically controlled throttle control unit having the opener-side function to hold the throttle valve at the intermediate position in the case where the electric current to the drive motor is stopped for some reason, however, two lever members including an opener member and an intermediate stopper member and two spring members including a spring for the opener-side function and a spring for a return-side function are required. The larger number of component parts thus required inconveniently results in a high cost. Also, the use of a complicated configuration, wherein the intermediate stopper member in contact with the engaging portion of the throttle body sets the throttle valve at the intermediate position through a contact portion with the opener member, inconveniently changes the default opening degree of the throttle valve due to variations in parts tolerances.
To obviate these inconveniences, a single-coil spring structure for an opener mechanism of an electronically controlled throttle control unit having a simplified configuration with a fewer component parts has been proposed wherein, for the purpose of improving the opening degree position accuracy of the throttle valve at the intermediate position (also called the default position), as shown in FIG. 14, the coupling between a return spring 101 having the return-side function (hereinafter referred to as the return spring function) and a default spring 102 having the opener-side function (hereinafter referred to as the default spring function) is bent in the shape of a substantially inverted-U into a U-shaped hook portion 103, and the ends of the single-coil spring structure (including a first end of the return spring 101 and a second end of the default spring 102) are wound in different directions (see, for example, Japanese Unexamined Patent Publication No. 2002-256894, pp. 1–10, FIGS. 1 to 7) and Japanese Unexamined Patent Publication No. 2004-144039, pp. 1–15, FIGS. 1 to 5).
In the electronically controlled throttle control units described in the two patent publications described above, a power unit for driving a throttle valve 108 and a throttle shaft 109 accommodated openably in a throttle bore 105 of a throttle body 104 is configured of a drive motor 110 making up a drive source and a mechanical reduction gear for reducing the rotational speed of the drive motor 110 to a predetermined reduction ratio. The mechanical reduction gear includes a pinion 111 fixed on the motor shaft of the drive motor 110, an intermediate reduction gear 112 rotated in mesh with the pinion 111 and a valve gear 113 rotated in mesh with the intermediate reduction gear 112. The valve gear 113 is integrally coupled with a first axial end of the throttle shaft 109. Between the closed-up position (idle opening) and the intermediate position (default opening) of the throttle valve 108, the U-shaped hook portion 103 of a single coil spring is formed to engage the forward end of an intermediate opening degree adjustment screw 115 forced into a boss-shaped intermediate position stopper 114 formed integrally with the outer wall of the gear housing 107 of the throttle body 104. Reference numeral 121 designates a body-side hook, numeral 122 a gear-side hook, numeral 123 an anti-horizontal displacement guide, numeral 124 an engaging portion formed on the opener member 106, numeral 125 a body-side spring inner peripheral guide, and numeral 126 a gear-side spring inner peripheral guide.
In the electronically controlled throttle control units described in the two Japanese patent publications described above, however, the intermediate position stopper 114 of the gear housing 107 of the throttle body 104 includes the intermediate opening adjustment screw 115 for adjusting the intermediate position (default opening degree) of the throttle valve 108.
For the purpose of reducing the fuel consumption, weight and costs, as shown in FIGS. 9 to 13, the throttle body 104 and the sensor cover 127 are formed of resin, i.e. the throttle body 104 and the sensor cover 127 are formed of a resin material integrally with each other. A base hole (guide hole) having a circular cross section is formed in advance to guide the intermediate opening degree adjustment screw (tapping screw) 115 along the fastening direction, after which a fastening tool (hexagon nut) is engaged with a hexagon hole 116 at the head of the intermediate opening degree adjustment screw 115. In this way, the intermediate opening degree adjustment screw 115 is forced into the intermediate position stopper 114 by fastening (tapping) it to the intermediate position stopper 114. This process requires a very large fastening torque to be applied to the intermediate opening degree adjustment screw 115 in view of the fact that the diameter of the male screw of the intermediate opening degree adjustment screw 115 is larger than the inner diameter of the base hole and that the intermediate position stopper 114 has a large thickness to secure at least a certain degree of rigidity and strength.
As a result, the stress (internal distortion) caused by fastening (tapping) the intermediate opening degree adjustment screw 115 is left in the neighborhood of the base hole of the intermediate position stopper 114 of the gear housing 107, thereby sometimes reducing the rigidity and strength of the intermediate position stopper 114. Also, the resultant creep often reduces the axial fastening force of and loosens the intermediate opening degree adjustment screw 115. Further, each time the engine is stopped and power to the drive motor is switched off, a shock load from the U-shaped hook 103 of the coil spring is repeatedly exerted on the forward end corners of the intermediate opening degree adjustment screw 115 when the throttle valve 108 returns to the intermediate position (default opening degree) or when the opening of the throttle valve 108 is between the closed-up position (idle opening degree) and the intermediate position (default opening degree). Therefore, the intermediate position stopper 114 cannot firmly hold and fix the intermediate opening degree adjustment screw 115, thereby posing the problem that the throttle opening degree adjusting function (especially, the intermediate opening degree adjusting function) is adversely affected.